1. Field of the Invention
Embodiments of present invention generally relate to an encapsulation structure comprising a luminescent wavelength conversion material for solar cells, solar panels, or photovoltaic devices, which enhances the solar harvesting efficiency of these devices.
2. Description of the Related Art
The utilization of solar energy offers a promising alternative energy source to the traditional fossil fuels, and therefore, the development of devices that can convert solar energy into electricity, such as photovoltaic devices (also known as solar cells), has drawn significant attention in recent years. Several different types of mature photovoltaic devices have been developed, including a Silicon based device, a III-V and II-VI PN junction device, a Copper-Indium-Gallium-Selenium (CIGS) thin film device, an organic sensitizer device, an organic thin film device, and a Cadmium Sulfide/Cadmium Telluride (CdS/CdTe) thin film device, to name a few. More detail on these devices can be found in the literature, such as Lin et al., “High Photoelectric Conversion Efficiency of Metal Phthalocyanine/Fullerene Heterojunction Photovoltaic Device” (International Journal of Molecular Sciences 2011). However, the photoelectric conversion efficiency of many of these devices still has room for improvement and development of techniques to improve this efficiency has been an ongoing challenge for many researchers.
Recently, one technique developed to improve the efficiency of photovoltaic devices is to utilize a wavelength down-shifting film Many of the photovoltaic devices are unable to effectively utilize the entire spectrum of light as the materials on the device absorb certain wavelengths of light (typically the shorter UV wavelengths) instead of allowing the light to pass through to the photoconductive material layer where it is converted into electricity. Application of a wavelength down-shifting film absorbs the shorter wavelength photons and re-emits them at more favorable longer wavelengths, which can then be absorbed by the photoconductive layer in the device, and converted into electricity.
This phenomenon is often observed in the thin film CdS/CdTe and CIGS solar cells which both use CdS as the window layer. The low cost and high efficiency of these thin film solar cells has drawn significant attention in recent years, with typical commercial cells having photoelectric conversion efficiencies of 10-16%. However, one issue with these devices is the energy gap of CdS, approximately 2.41 eV, which causes light at wavelengths below 514 nm to be absorbed by CdS instead of passing through to the photoconductive layer where it can be converted into energy. This inability to utilize the entire spectrum of light effectively reduces the overall photoelectric conversion efficiency of the device.
There have been numerous reports disclosing the utilization of a wavelength down-shifting material to improve the performance of photovoltaic devices. For example, U.S. Patent Application Publication No. 2009/0151785 discloses a silicon based solar cell which contains a wavelength down-shifting inorganic phosphor material. U.S. Patent Application Publication No. US 2011/0011455 discloses an integrated solar cell comprising a plasmonic layer, a wavelength conversion layer, and a photovoltaic layer. U.S. Pat. No. 7,791,157 discloses a solar cell with a wavelength conversion layer containing a quantum dot compound. U.S. Patent Application Publication No. 2010/0294339 discloses an integrated photovoltaic device containing a luminescent down-shifting material, however no example embodiments were constructed. U.S. Patent Application Publication No. 2010/0012183 discloses a thin film solar cell with a wavelength down-shifting photo-luminescent medium; however, no examples are provided. U.S. Patent Application Publication No. 2008/0236667 discloses an enhanced spectrum conversion film made in the form of a thin film polymer comprising an inorganic fluorescent powder. However, each of these patents and patent application publications, which are incorporated herein by reference in their entirety, apply this additional wavelength conversion layer directly on top of or into the solar cell device, which adds additional cost to manufacturing the devices, and also increases the device thickness which can lead to an increase in the loss of photons to the environment out of the side surfaces.
Furthermore, solar modules are traditionally mounted outdoors on rooftops or in wide-open spaces where they can maximize their exposure to sunlight, see U.S. Patent Application Publication No. 2007/0295388, which is hereby incorporated by reference in its entirety. This type of outdoor placement subjects these devices to constant weather and moisture exposure, and therefore, they must have sufficient protection to provide many years of stable operation. Traditionally, solar cell modules have been weatherproofed by using glass sheets, which is expensive, heavy, and rigid, and also requires some type of edge tape to prevent moisture from penetrating the sides. U.S. Pat. No. 7,976,750 discloses a method of encapsulating solar modules by embedding them between two polymer layers and then filling in the gaps with a foaming agent. U.S. Patent Application Publication No. 2011/0017268 discloses a nanostructured polymer material for encapsulating solar module devices. U.S. Pat. No. 7,943,845 discloses a method of encapsulating solar modules using a poly(vinyl butyral) composition. However, none of these patents and patent application publications, which are incorporated herein by reference in their entirety, attempt to simultaneously provide environmental protection for the solar devices as well as enhanced solar harvesting efficiency.